This invention is related to III-nitride based high electron mobility transistors (HEMT). Nitride materials include GaN, InN, and AlN, as well as the alloy materials such as AlGaN, InAlN, InGaN, etc. Due to their unique material properties, nitride materials especially AlGaN and InAlN are particularly suitable for HEMT devices capable of delivering high frequency, high power. The nitride based HEMT has found its applications in areas of mobile, satellite, radar communications, and proven its advantages over other semiconductor materials such as Si or GaAs.
The fabrication of GaN HEMT starts with epitaxy of nitride materials on substrates (typically SiC, Si, Sapphire, or GaN, etc) with metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). The typical epitaxy structure of nitride HEMT comprises of a nucleation layer (typically AlN or low-temperature GaN), a highly-resistive GaN template and barrier layer (such as AlGaN or AlInN). The epi wafer will be tested for essential materials properties. Once the epi wafer is characterized, and it will go through wafer fab process to form HEMT devices. Typically, ohmic contact metals (Ti/Al/Ni/Au) will be deposited on wafers and annealed to form ohmic contact with nitride materials. Then gate area is defined by typical photolithography process, and gate metal is subsequently deposited to the gate area to form gate contact. More advanced HEMT technologies will have different variations from the above-mentioned process. For example, technologies such as in-situ SiNx passivation, multiple 2DEG channels, different barrier materials, device isolation by dry etching, device passivation, field plate will be used for further improvement of device improvement.
One of the key factors for achieving high-performance radio-frequency (RF) performance of HEMT device is to minimize RF dispersion. The RF dispersion manifest itself as the discrepancy between the maximum channel current as well as knee voltage under microwave frequency and DC condition. Evidences have been shown that the dispersion is strongly related to surface-state charges. Surface passivation with SiNx films could partially mitigate the issue without completely eliminating it. In addition, SiNx passivation effect is very sensitive to both surface and SiNx deposition condition, therefore resulting in poor reproducibility and repeatability. In addition, the RF dispersion issue becomes more severe once the barrier thickness is reduced (i.e., for higher frequency applications), which brings the electron in 2DEG closer to surface, therefore the RF dispersion issue requires a different approach. As an alternative effort to address the RF dispersion issue, Oleg M. eta [i] reported a Si-doped AlGaN barrier could reduce RF dispersion at the price of an increased gate leakage, which essentially solves one problem yet creates another worse one. The current solution needs improvement. Here in the present invention, we disclose a method to fabricate dispersion-free nitride HEMT devices without a significant increase of gate leakage.